As a measure for stabilizing the behavior of a vehicle, various forms of rear wheel toe angle control devices (RTCs) have been proposed, and the toe angles of the right and left rear wheels are steered symmetrically (in opposite phase) or asymmetrically (in same phase) so that the vehicle may be given with a desired dynamic property. A control device disclosed in Japanese patent No. 3179271 stabilizes the behavior of the vehicle by using a four wheel steering (4WS) device that sets a target of bringing a lateral slip angle of the vehicle to zero or a prescribed value. Various control processes for such a device have also been proposed, and one of them changes a steering angle ratio between the front and rear wheels depending on the vehicle speed (opposite phase in a low speed range and same phase in a high speed range). Such a control process, be it an RTC or 4WS, basically relies on a feed forward control that minimizes a deviation of a yaw rate response of an actual vehicle model to a steering input from a yaw response of an ideal vehicle model to the same steering input.
FIG. 1 shows an example of such a RTC device. As a front wheel steering angle δf is inputted to both an actual vehicle model (Gγ0(s)) 11 and an ideal vehicle model (Gideal(s)) 10, a deviation between the outputs of the two models is forwarded to a RTC feed forward transfer (F/F) function property (P(s)) 12. An output of the RTC F/F function property (P(s)) 12 acts upon a vehicle 13 as an added yaw moment, and an integrated value of the sum of the yaw moment of the vehicle owing to a front wheel steering angle and the added yaw moment is given as a cause of an actual yaw rate of the vehicle. By using a feed forward control process using the behavior of the ideal vehicle model as a reference, the driving stability of the vehicle can be improved and a high responsiveness can be achieved at the same time.
In this case, as disclosed in WO08/047,481, the motion of equation of a vehicle having a transfer function property of δr=Gr·δf can be given by the following equation.
            [                                                                  m                ·                V                ·                s                            +                              (                                                      K                    f                                    +                                      K                    r                                                  )                                                                                        m                ⁢                                                                  ⁢                V                            +                                                                                          l                      f                                        ·                                          K                      f                                                        -                                                            l                      r                                        ·                                          K                      r                                                                      V                                                                                                                          l                  f                                ·                                  K                  f                                            -                                                l                  r                                ·                                  K                  r                                                                                                        I                ·                s                            +                                                                                          l                      f                      2                                        ·                                          K                      f                                                        -                                                            l                      r                      2                                        ·                                          K                      r                                                                      V                                                        ]        ·          [                                    β                                                γ                              ]        =                         [                                                                              K                  f                                +                                  Gr                  ·                                      K                    r                                                                                                                                                                l                    f                                    ·                                      K                    f                                                  -                                  Gr                  ·                                      l                    r                                    ·                                      K                    r                                                                                      ]            ·              δ        f            The steering angle δr of the rear wheel can be given by the following equation.
                              δ          r                =                                            -                                                                                                                  m                        ·                                                  l                          f                                                                                                                      k                          r                                                ·                        l                                                              ⁢                                          V                      ·                      s                                                        +                  1                                                                                                                    m                        ·                                                  l                          r                                                                                                                      k                          f                                                ·                        l                                                              ⁢                                          V                      ·                      s                                                        +                  1                                                      ·                          1                              G                γ0                                              ⁢                                    (                                                G                  ideal                                -                                  G                  γ0                                            )                        ·                          δ              f                                                          (        1        )            where m: vehicle mass, 1: wheel base, 1f and 1r: distances of front and rear axles from the gravitational center, δf: front wheel steering angle, Kf and Kr: cornering powers of front and rear wheels, and V: vehicle speed. Gideal represents the ideal vehicle model, and Gγ0 represents the actual vehicle model when δr=0 in FIG. 1.
Because a vehicle is typically considered to be easy to drive when the delay of the yaw rate to a steering, input is small, and the response speed is high, it is preferable to determine Gideal such that the resonance gain for the response Gγ0 when no control is made is relatively low, and raise the frequency range that is damped (thereby maintain a high tracking performance up to a high frequency range) to a higher range.
However, a skilled vehicle operator is able to compare the response of a vehicle to a given steering input to a reference vehicle which is formed from past experience, and is therefore able to quickly grasp the relationship between the control input and resulting response. Therefore, a skilled vehicle operator is able to handle the vehicle even when the response of the vehicle is highly prompt, and tends to prefer a vehicle having a quick response. On the other hand, an unskilled vehicle operator is unable to predict the response of the vehicle to a given steering input, and decides how to operate the steering wheel after observing how the vehicle is responding to a steering input. Therefore, an unskilled vehicle operator tends to excessively turn the steering wheel, and often reverses a steering action while making a turn to compensate for excessive turning of the steering wheel.
Therefore, when a device that can affect the driveability of the vehicle such as an RTC device is designed according to a certain design criterion, it does not necessarily satisfy all the users. Therefore, some compromise is unavoidable in determining the ideal vehicle model Gideal for such a device, and the final result is often unsatisfactory to skilled vehicle operators. In particular, a prompt response of a vehicle to a steering input is desired in a situation when avoiding a dangerous situation. Also, as the skill of a vehicle operator advances over time, the vehicle operator tends to prefer a vehicle demonstrating a quick response more and more.